Generally, in a semiconductor device fabrication process, a layer or thin film is formed using a chemical vapor deposition method, a sputtering method and/or an atomic layer deposition (ALD) method, etc. The chemical vapor deposition method has an advantage in that the thin film can be formed to have excellent step coverage, but has a disadvantage in that the process temperature may be high and it may be difficult to control the thickness of the thin layer. By comparison, the sputtering method has an advantage in that the process temperature is low and a material layer can be formed without developing a precursor as may be required for vapor deposition. However, the sputtering method has a disadvantage in that step coverage may be poor.
Because the atomic layer deposition method can form an atomic monolayer at a low temperature, it can advantageously provide a layer having a low thickness, a uniform surface and excellent step coverage. Generally, the atomic layer deposition method employs a cycle including: a first feed step of feeding a first reaction gas into a reaction chamber to form a first reactant in the form of an atomic layer on a substrate; a first purge step of purging an internal section of the reaction chamber; a second feed step of feeding a second reaction gas into the reaction chamber to provide a chemical exchange reaction with the first reactant of the atomic layer; and a second purge step of purging the internal section of the reaction chamber. This cycle can be performed repeatedly to control the thickness of the layer.
In the atomic layer deposition method, because several atomic layers are laminated to form the desired layer, it is typical to feed the reaction gas in an amount sufficient to form the atomic layer. Further, because the precursor used for the reaction gas is expensive, the amount of the reaction gas used should be minimized. Because the atomic layer deposition method forms the atomic layer on a surface of the substrate by the chemi-substitution-reaction, it is necessary, before the reaction gas is fed, to completely purge the reactants incompletely physisorbed on the surface of the substrate from the internal section of the reaction chamber.
FIG. 1 is a flow chart representing a conventional layer deposition method. FIG. 2 is a timing diagram representing a conventional layer deposition method. Referring to FIGS. 1 and 2, in the conventional layer deposition method, a cycle for forming a layer is repeatedly performed, the cycle including the steps of: a first feed step t1, 10 for feeding the first reactant; a first purge step t2, 20 for purging the reaction chamber; a second feed step t3, 30 for feeding a second reactant; and a second purge step t4, 40 for purging the reaction chamber.
In the first feed step t1, 10 for feeding the first reactant into the reaction chamber, a first flow of an inert gas is used as a carrier gas and the first reactant is chemisorbed or physisorbed on the substrate. In the first purge step t2, 20, the first flow of the inert gas is fed to purge or remove the first reactant within the reaction chamber, within the first reactant feed lines, and physisorbed on the substrate. In the second feed step t3, 30, the second reactant is fed into the reaction chamber using the second flow of the inert gas as the carrier gas, and the first reactant chemisorbed on the substrate is chemically exchanged with the second reactant to form the layer. Thereafter, in the second purge step t4, 40, a second flow of an inert gas is fed to purge the reaction chamber and the second reactant feed line. The second reactant remaining in the reaction chamber and the second reactant feed line is removed along with the second reactant that is physisorbed on the substrate without chemi-substitution-reacting with the first reactant.
In the atomic layer deposition method, inert gas is used as the carrier gas and the purge gas. Accordingly, the amount of the reactant used as the precursor can be reduced by reducing the flow of the inert gas. The feed amount of the inert gas is controlled by a mass flow controller (MFC). In a general atomic layer deposition process, the feed step and the purge step are performed for a very short time of about one second, whereas the control response time of the mass flow controller may be the same as or longer than the process time for the feed step and the purge step. If the reactant feed line and the reaction chamber are not sufficiently purged in the purge step, not only may a non-uniform layer be formed, but the layer may also be formed so as to contain a non-required element or impurity. Accordingly, in order to purge the reactant feed line and the reaction chamber, a large amount of purge gas should be fed. However, as described above, because the mass flow controller cannot adjust the flow within the short time range allowed, if the amount of reactant consumed is reduced, the purge gas cannot be sufficiently fed in the purge step.
As a result, the conventional atomic layer deposition method has a drawback in that there is a limitation in reducing the amount of the reactant consumed. If the amount of the reactant consumed is reduced, the purging may not be performed sufficiently or smoothly.